(1) Field of the Invention
The present invention relates to alkaline storage cells such as nickel-cadmium storage cells which employ a paste type cadmium electrode as their negative electrode, and to manufacturing methods therefor.
(2) DESCRIPTION OF THE PRIOR ART
A cadmium electrode of the paste type which requires a relatively simple manufacturing process and low manufacturing cost is in wide industrial use as the cadmium electrode for alkaline storage cells such as nickel-cadmium cells. Generally, the paste type cadmium electrode is manufactured through a series of operations which include kneading powder of an active material such as cadmium oxide or cadmium hydroxide with reinforcing fibers for providing the electrode with good strength, a binder such as polyvinyl alcohol for binding these materials, and water or any other suitable solvent to form an active material paste, coating the paste on surfaces of a conductive support, drying the paste, carrying out an electrochemical formation of the electrode as desired, and cutting the product to a size suited to the type of cell for which the electrode is intended. When the active material paste is prepared by adding the binder and water to cadmium oxide powder, cadmium oxide reacts with water and changes into cadmium hydroxide thereby expediting hardening of the active material paste. This presents serious operational difficulties to the manufacture of cells, and impedes production of cells having uniform quality. In order to prevent cadmium oxide from changing into cadmium hydroxide during the cell manufacturing process, phosphate, silicate or the like is addd to the active material paste formed of cadmium oxide, binder and water (as disclosed in Japanese Patent Publication No. 58-48990). In another measure taken, an organic solvent is used instead of water for preventing hardening of the active material due to the hydration of cadmium oxide. These measures are effective to avoid the operational difficulties due to the hardening of the active material paste, and to provide the advantage of increasing the rate of filling the active material by a rolling operation in that the active layer paste is maintained soft.
However, the soft active material layer has a disadvantage when used for forming a spiral wound electrode. Particularly where pressure rollers or the like are used to wind the electrode to form a spiral wound electrode, the pressure applied to the electrode increases progressively from the beginning to the end of the winding. As a result, the unhydrated cadmium electrode has its thickness reduced substantially linearly from the portion wound at the beginning to the portion wound at the end, which accompanies a reduction in the porosity of the electrode. It is thus extremely difficult to form a spiral wound electrode with the cadmium electrode while maintaining uniform porosity longitudinally of the electrode.
On the other hand, where the cadmium electrodes of the same thickness are used, the unhydrated cadmium electrode generally has a lower average porosity than the electrochemically formed cadmium electrode having cadmium oxide essentially changed into cadmium hydroxide in the course of the electrochemical formation. Thus, where the cell employing the unhydrated cadmium electrode is filled with the same amount of electrolyte as the cell employing the electrochemically formed cadmium electrode, the unhydrated cadmium electrode is capable of holding a smaller amount of electrolyte because of the lower average porosity and, therefore, the former cell has the electrolyte in the greater amount between the positive and negative electrodes. With the type of cell in which oxygen gas generated by the positive electrode at charging times is absorbed by the negative electrode, there is a proper level for the amount of electrolyte present between the positive and negative electrodes. An amount of electrolyte exceeding this level will prevent the oxygen gas from reaching the negative electrode. The excessive amount of electrolyte lowers the oxygen gas consumption capability of the cadmium electrode especially during initial charge and discharge cycles immediately following cell fabrication. Accordingly, the electrolyte must be injected into the cell in a limited amount in order that the oxygen gas consumption capability be above a certain level. With progress of the charge and discharge cycles, electrically charged inactive metallic cadmium increases and accumulates in the cadmium electrode. Consequently, the electrode has an increased average porosity and holds an increased amount of electrolyte thereby to reduce the amount of electrolyte present between the positive and negative electrodes, which leads to deterioration in discharge characteristics of the cell. This situation may be avoided if the cell is formed by the aforesaid electrochemical process, but the electrochemical formation requires extensive equipment, increased processing steps and high manufacturing cost. Thus, a truly satisfactory method of manufacturing alkaline storage cells has not been developed yet.
Further, when a spiral wound electrode assembly is formed by using the paste type cadmium electrode as the negative electrode, a separator is disposed around the outermost periphery of the electrode assembly to protect the cadmium electrode. This separator fixed with a tape is disposed to facilitate insertion of the electrode assembly to the can.
Proposals have been made in recent years to dispense with such a separator for high energy density of the cell. The paste type negative cadmium electrode, however, has an active material layer of low mechanical strength, and the active material tends to fall easily. Thus, there arises the inconvenience of the soft active material layer becoming peeled off when th spiral wound electrode assembly is inserted into the can. This is a problem peculiar to the paste type negative cadmium electrode not subjected to the electrochemical formation.
Where a cadmium electrode of the sintered type having a solid surface is used, it is possible to dispense with the separator disposed peripherally of the electrode assembly. However, the sintered type, compared with the paste type, has the disadvantages of complicated manufacturing process and high manufacturing cost.